1. Field of the Invention
The present invention relates to a filter having a directional coupler for use in microwave communication, and more particularly to a filter containing a directional coupler therein, a composite filter device and a communication device each including the same.
2. Description of the Related Art
Generally, a filter is disposed at the first stage in a communication device, and to check the operation of the communication device, a directional coupler is provided. FIG. 1 is a block diagram of such a communication device such as a portable telephone or the like.
Referring to FIG. 1, a power amplifier power-amplifies a transmission signal, and a low-pass filter attenuates the higher harmonics of the signal. A directional coupler outputs a part of the transmission signal to the antenna transmission power monitor. The antenna transmission power monitor detects the input signal and adjusts the output of the power amplifier, which is transmitted to the antenna via the directional coupler. Thus, the output of the antenna to be radiated externally is continuously stabilized.
Such methods of designing filters for use in microwave communication as described above are known. For example, a low-pass filter using a coaxial line, a comb line filter, a waveguide filter, and so forth are described in: Matthaei and others, xe2x80x9cMicrowave Filters, Impedance-Matching and Networks, and Coupling Structurexe2x80x9d, Artech House Co. Moreover, methods of designing a low-pass filter and a band-pass filter using microstrip lines are described in Konishi, xe2x80x9cDesign and Application of Filter Circuit for Communicationxe2x80x9d, Sougou-Denshi Shuppan (1994).
FIGS. 2 and 3 show typical low-pass filters produced by the above-mentioned methods.
FIG. 2 is an exploded perspective view of a low-pass filter using a coaxial line. FIG. 3 is a perspective view of a microstrip type low-pass filter.
The low-pass filter shown in FIG. 2 comprises an inner conductor 103 arranged in an outer conductor 104. The inner conductor 103 comprises high impedance portions 101 and low impedance portions 102 alternately connected to each other. In each high impedance portion 101, the size of a plane perpendicular to the signal propagation direction is small and the axial length is large. In each low impedance potion 102, the size of a plane perpendicular to the signal propagation direction is large and the axial length is small.
The low-pass filter shown in FIG. 3 contains a line electrode 107 formed on the front surface of a dielectric substrate 108 and a ground electrode 109 formed on the back surface of the dielectric substrate 108. The line electrode 107 comprises high impedance portions 105 and low impedance portions 106 which are alternately arranged. For each high impedance portion 105, the width with respect to the signal propagation direction is small, and the length is large. For each low impedance portion 106, the width is large, and the length is small.
Since the high impedance portions and the low impedance portions are alternately arranged as described above, the high and low impedance portions function as inductors and capacitors, respectively. FIG. 4 is an equivalent circuit diagram of the above-described low-pass filter. Thus, the low-pass filter comprising a multi-stage LC ladder circuit is formed.
Techniques for designing directional couplers are described in xe2x80x9cMicrowave Circuit for Communicationxe2x80x9d, The Institute of Electronics, Information, and Communication Engineers (1981). FIGS. 5 and 6 show well-known typical structures of the couplers.
FIG. 5 is a schematic view of a hybrid circuit. FIG. 6 is a schematic view of a transverse coupling type directional coupler.
In the hybrid circuit shown in FIG. 5, a main line 111 is formed on the front surface of a dielectric substrate 110, and a ground electrode 112 is formed on the opposite surface of the substrate 110. The lengths of the line portions 111a to 111d of the main line 111 are set to be equal to a quarter of the wavelength of a transmission signal, respectively, so that the characteristic impedances of the respective lines can be matched with each other.
Moreover, the transverse coupling type directional coupler shown in FIG. 6 contains a distributed coupling line in which a main line 114a and a coupling line 114b adjacent to the main line 114a are formed on the front surface of a dielectric substrate 113 which has a ground electrode 115 formed on the back surface thereof. The smaller the line length of the coupling portion becomes, the more the directivity decreases. A superior directivity can be attained by setting the line length at a quarter of the wavelength of a transmission signal.
It is generally known that to increase the width of the frequency band in which the directivity can be attained, line conductors in a coupling portion have a multistage structure. FIG. 7 shows a transverse coupling type directional coupler having the above-described multistage structure. In FIG. 7, a dielectric substrate 116, a main line 117a, a coupling line 117b, and a ground electrode 118 are shown.
For the transverse coupling type directional coupler, the coupling degree has a limitation since the size is regulated. Thus, according to the structure shown in FIGS. 8A and 8B, coupling degree adjusting conductors 121a and 121b are arranged on a coupling portion so as to sandwich a dielectric. In FIGS. 8A and 8B, a dielectric substrate 119, a main line 120a, a coupling line 120b, and a ground electrode 122 are shown. The first layer formed on the dielectric substrate 119 is the same as the circuit shown in FIG. 6.
Communication devices provided with the above described filters and directional couplers still have the following problems.
In particular, a filter and a directional coupler are separately formed in the prior art communication devices. Thus, the size of the device is increased. Moreover, since a signal is transmitted via the two elements, the number of sites in which loss is generated when a signal passes the sites is increased. Thus, as a whole, the transmission loss is increased.
To solve the above-described problem, a method for forming a filter and a directional coupler on the same substrate or in the same case has been devised and disclosed.
Examples of such method are disclosed in Japanese Unexamined Patent Application Publication No. 6-120708, Japanese Unexamined Patent Application Publication No. 9-270732, Japanese Unexamined Patent Application Publication No. 11-220312, and Japanese Unexamined Patent Application Publication No. 2001-94315.
As described in Japanese Unexamined Patent Application Publication No. 6-120708, resonators constituting a filter and input-output terminals are connected to lines, respectively. A coupling line is formed adjacent to the 4 transmission lines to produce a directional coupler.
According to Japanese Unexamined Patent Application Publication No. 9-270732, a coupling line is arranged adjacent to a transmission line which constitutes a band-pass filter, formed on a dielectric substrate, as a demultiplexer, whereby a directional coupler is formed.
According to Japanese Unexamined Patent Application Publication No. 11-220312, a coupling line is arranged in the position where the line is to be coupled to the coil pattern portion of a low pass filter which is made of inner electrodes in a laminated multi-layer substrate, and is coupled to the coil pattern portion, whereby a directional coupler is formed.
According to Japanese Unexamined Patent Application Publication No. 2001-94315, a directional coupler comprises two coupling lines adjacent to each other. Lines which function as capacitors are arranged at both the ends of a main line of the coupling lines, so that the main line operates as an inductor. Thus, a low-pass filter is formed.
In the case of these integral devices comprising the directional couplers and the filters, a coupling line is arranged so as to be coupled to a transmission line which constitutes a filter, whereby a directional coupler is formed. For this configuration, a component which can constitute the coupling line in the filter is required. Moreover, a sufficient length must be ensured for the coupling portion to attain a coupling degree which provides a sufficient directivity in the case of a transverse coupling type directional coupler. When the transverse coupling type directional coupler is combined with a low-pass filter comprising pattern electrodes, the length of the line constituting the low-pass filter becomes shorter than a quarter of the wavelength of a transmission signal. Therefore, the length of the coupling line is insufficient, and thus, the directivity which can be attained has a limit.
Moreover, problems are caused in that the directivity characteristic or the like is difficult to control when the electrical length of the coupling line is short.
In band-pass filters, the structure in which a band-pass characteristic is attained by use of the coupling between the resonators constituting a filter is predominantly employed. These devices have no main lines. Accordingly, a directional coupler using a coupling line system can not be formed between resonators.
Moreover, the structure in which a resonator is connected to a line having a length equal to a quarter of the wavelength of a transmission signal is dominantly employed in band-stop filters. However, a superior directivity can not be obtained even if a coupling line comprising simple parallel two conductors is provided, since a complicated standing-wave is generated inside of the filter.
Accordingly, it is an object of the present invention to provide a filter having a directional coupler which has a simple structure, a good coupling characteristic and a superior directivity, a composite filter device, and a communication device, and to provide a method of adjusting the directional coupler.
To achieve the above-described object, according to the present invention, a part of a transmission signal is picked up at plural sites in the filter. The phases of the signals are controlled and synthesized by means of a circuit pattern to realize the characteristics with which the directional coupler performs its function with respect to input or output of the filter. According to this configuration, the filter is not required to contain a transmission line portion. The phases of the picked-up signals are controlled and synthesized, and thereby, the directivity characteristic at a desired frequency is enhanced while influences upon the filter characteristic are suppressed.
The principle of this configuration will be described with reference to FIG. 9 which is an equivalent circuit diagram of a filter having a directional coupler.
In FIG. 9, a filter 150 and a directional coupler 151 are shown. The filter 150 is provided with two input-output terminals, that is, a port 1 and a port 2 each of which can function as an input and/or output terminal. Each of the ports 1 and 2 comprise at least two resonators having a characteristic impedance Z. On the other hand, the directional coupler 151 is provided with two external input-output terminals, that is, a port 3 and a port 4, and is coupled to the filter via ports A and B. The electrical angles of the lines between the port A and the port 3, between the port B and the port 4, and between the port 3 and the port 4 are represented by xcex8a, xcex8b, and xcex80, respectively.
In this circuit, a transmission signal is picked up via two sites, that is, the ports A and B. The signals transmitted via the port 1 and the ports A and B reach the port 3 to overlap each other, giving a large signal, while the signals reaching the port 4 are canceled by each other. In this case, the circuit functions as a directional coupler. Needless to say, when the signals reaching the port 4 overlap each other, and the signals reaching the port 3 are cancelled by each other, and the circuit also functions as a directional coupler. In particular, directivity can be attained by appropriately setting the intensities of the signals picked up at the ports A and B, the propagation phase of the line between the port A and the port 3, and the propagation phase of the line between the port B and the port 4. Therefore, it is unnecessary to set the phase difference between the transmission signals picked up at the ports A and B at xcfx80/2.
The coupling element constituting the port A comprises an appropriate combination of, e.g., a conductor loop, a line electrode connected to the conductor loop, a stub connected thereto, and the like. The propagation phase is adjusted by selection of materials and shapes for the loop, the length of the line electrode, and the shapes, sizes, and arrangement position of the stub.
This principle can be applied to a multi-stage configuration of the coupling portion. That is, the number of ports through which signals are picked up from the filter may be increased and combined with each other for the configuration.
In a practical circuit, the phase difference between signals at the ports A and B of a signal input via the port 1 is different from that between signals at the ports A and B of a signal input via the port 2. However, this problem can be solved by setting and combination of the line lengths in the directional coupler. Thus, superior directivity and coupling degree can be obtained.
For simple illustration of this principle, the following is assumed. That is, one half of a signal from the port A flows to the port 3, and the other half flows to the port 4. Moreover, one half of a signal from the port B flows to the port 3, and the other half flows to the port 4. The phase differences between the signals at the ports A and B is represented by xcex81 for a signal input via the port 1, and xe2x88x92xcex82 for a signal input via the port 2. The amplitudes are represented by 2W. It should be noted that the signs of xcex81 and xe2x88x92xcex82 are opposite, since the propagation directions are different from each other.
In the above-described configuration, a signal input via the port 1 and transmitted toward the port 3 side can be expressed as follows:
W sin(xcfx89txe2x88x92xcex81xe2x88x92xcex8a)+W sin(xcfx89txe2x88x92xcex8bxe2x88x92xcex80)=W cos{(xe2x88x92xcex81xe2x88x92xcex8a+xcex8b+xcex80)/2}sin{(2xcfx89txe2x88x92xcex81xe2x88x92xcex8axe2x88x92xcex8bxe2x88x92xcex80)/2}xe2x80x83xe2x80x83(1) 
On the other hand, a signal input via the port 1 and transmitted toward the port 4 side can be expressed as follows:
W sin(xcfx89txe2x88x92xcex81xe2x88x92xcex8axe2x88x92xcex80)+W sin(xcfx89txe2x88x92xcex8b)=W cos{(xe2x88x92xcex81xe2x88x92xcex8a+xcex8bxe2x88x92xcex80)/2}sin{(2xcfx89txe2x88x92xcex81xe2x88x92xcex8axe2x88x92xcex8b+xcex80)/2}xe2x80x83xe2x80x83(2) 
The sin terms in the equations (1) and (2) represent time-dependent changes, respectively. The cos terms represent the amplitudes and have a relation to the directivity and the coupling degree.
Accordingly, if cos{{(xe2x88x92xcex81xe2x88x92xcex8a+xcex8b+xcex80)/2} and cos{(xe2x88x92xcex81xe2x88x92xcex8a+xcex8bxe2x88x92xcex80)/2}become xc2x11 and 0, respectively, this means that the signals flow in one direction. That is, the phase difference between (xe2x88x92xcex81xe2x88x92xcex80) and (xe2x88x92xcex81+xcex80) becomes r. Accordingly, a directional coupler can be formed by setting (xe2x88x92xcex8a+xcex8b) at an appropriate value.
On the other hand, a signal input via the port 2 and transmitted toward the port 3 side can be expressed as follows:
W sin(xcfx89t+xcex82xe2x88x92xcex8a)+W sin(xcfx89txe2x88x92xcex8bxe2x88x92xcex80)=W cos{(+xcex82xe2x88x92xcex8a+xcex8b+xcex80)/2}sin{(2xcfx89t+xcex82xe2x88x92xcex8axe2x88x92xcex8bxe2x88x92xcex80)/1}xe2x80x83xe2x80x83(3) 
A signal input via the port 2 and transmitted toward the port 4 side can be expressed as follows:
W sin(xcfx89t+xcex82xe2x88x92xcex8axe2x88x92xcex80)+W sin(xcfx89txe2x88x92xcex8b)=W cos{(+xcex82xe2x88x92xcex8a+xcex8bxe2x88x92xcex80)/2}sin{(2xcfx89t+xcex82xe2x88x92xcex8axe2x88x92xcex8b+xcex80)/2}xe2x80x83xe2x80x83(4) 
Referring to these equations (3) and (4), if cos{(+xcex82xe2x88x92xcex8a+xcex8b+xcex80)/2} and cos{(+xcex82xe2x88x92xcex8a+xcex8bxe2x88x92xcex80)/2 become xc2x11 and 0, this means that the signals flow in one direction. That is, the phase difference between (+xcex82xe2x88x92xcex80) and (+xcex82+xcex80) becomes xcfx80. Accordingly, a directional coupler can be formed by setting (xe2x88x92xcex8a+xcex8b) at an appropriate value.
As seen in the above-description, a directional coupler can be formed, even if the interval between the pick-up positions is not limited to xcfx80/2.
According to the present invention, there is provided a directional coupler having a directional coupler which comprises at least two input-output terminals; at least two filter components; and a directional coupler comprising at least two coupling elements which are electromagnetically coupled to the filter components or a filter unit composed of the at least two filter components, a coupling line which electrically connects the at least two coupling elements to each other, and at least two coupling terminals electrically connected to the coupling line. Thus, the filter and the directional coupler are integrated with each other, and the transmission loss is reduced.
Preferably, the filter components include at least one of lumped constant elements, distributed constant lines, distributed constant resonators, plane circuits, wave guides, dielectric lines, dielectric resonators, and circuits composed of at least two laminated electrode-layers. Accordingly, the directional coupler and the filter are integrated with each other without using an especially complicated circuit.
Also, preferably, the coupling elements are ones selected from coupling probes disposed in the space defined by an inner conductor and an outer conductor or disposed in the vicinities of the filter components, coupling probes inserted into a metallic case, coupling electrode patterns formed on the surface of an insulation substrate, and reactance elements. Thus, coupling elements having a simple structure are coupled to the filter elements.
Preferably, at least one of the filter components is a multiple resonance mode element, and the coupling elements are arranged with respect to the multiple resonance mode element in such a manner that the coupling degrees for the respective resonance modes are different from each other. Accordingly, the directional coupler is integrated with the filter containing the multiple mode dielectric resonator.
Also, preferably, the number of the coupling elements is at least three, and at least one of the coupling elements is electrically connected to the coupling line in such a manner that the order in which the coupling elements are electrically connected to the coupling line is different from the order in which the coupling elements are arranged in the signal propagation direction. Accordingly, the design flexibilities of the directivity and the coupling degree are enhanced.
Preferably, the coupling elements are tip-open probes, or tip-loop probes electromagnetically connected to a ground conductor or the metallic case. Thus, the circuit can be formed irrespective of the shapes and sizes of the probes.
Further, preferably, the filter components include a capacitor comprising conductor patterns formed on the surface of an insulation substrate or comprising plural conductors arranged in the metallic case. Thus, the capacitor as the filter component can be easily formed.
Also, preferably, the coupling probes include at least one lead wire, sheet metal, coupling electrode pattern formed on the surface of an insulation substrate, coaxial line, microstrip line, and screw-shaped conductor. Thus, coupling elements each having a simple structure and a small size can be produced.
Preferably, the coupling elements or the coupling lines are provided with stub elements or reactance elements for adjusting the coupling characteristics. Thereby, the design flexibilities of the directivity and the coupling degree are enhanced.
Also, preferably, the coupling line comprises at least two line elements having different characteristic impedances. Thus, the design flexibility of the coupling line is enhanced, and the filter having a directional coupler can be easily formed.
Further, preferably, each stub element is formed so as to have a length equal to a quarter of the wavelength of the first harmonic of a transmission signal. Thus, the directivity and the coupling degree can be appropriately set. Superior directivity and coupling degree can be attained.
Also, preferably, a coupling line is arranged outside of the filter so as to be electromagnetically shielded from the filter components. Thus, the influence of a signal being transmitted through the filter upon the coupling line is suppressed.
Preferably, at least a part of the coupling line is arranged inside of the filter. Thus, the overall size of the filter having a directional coupling is reduced.
Preferably, the Metallic case is provided with holes through which members for mechanically changing the coupling elements or the coupling line are inserted inward of the metallic case. Therefore, the characteristics can be changed after construction.
Also, preferably, the metallic case is provided with screws for adjusting the characteristics of the coupling elements or the coupling line. Thus, the characteristics can be easily adjusted.
Preferably, the coupling line is provided at at least one end thereof with an attenuation circuit for attenuating an undesired mode signal excited in the coupling line. Thus, undesired signal is eliminated. Superior characteristics can be obtained.
Also, preferably, the attenuation circuit includes at least one resistor which is a variable resistor. Accordingly, the constants of the attenuation circuit can be easily changed, so that appropriate characteristics are attained.
Preferably, the coupling line has a resistor for termination connected at least one end thereof. Thus, the termination can be adequately performed, and superior transmission characteristics can be obtained.
Also, according to the present invention, the position and arrangement of the coupling probes or the shapes and sizes thereof are changed to adjust the coupling characteristic of the directional coupler. Thus, the coupling characteristic can be easily adjusted.
Preferably, the shape and size, position, and arrangement of the coupling line or the stub are changed, or a conductor or dielectric is connected to or positioned adjacent to the filter components to adjust the coupling characteristic of the directional coupler. Thus, the coupling characteristic can be easily adjusted.
Also, preferably, the length of each screw which is effective in coupling is changed so that the electromagnetic coupling degree between the filter components and the coupling element is adjusted. Thus, the electromagnetical coupling degree between the filter components and the coupling elements can be easily adjusted.
Preferably, in a composite filter device in accordance with the present invention, at least one of the filters thereof comprises the above-described filter having a directional coupler. Accordingly, a composite filter device having superior directivity and coupling degree, reduced transmission loss, and a simple structure is easily formed.
Preferably, a communication device in accordance with the present invention includes the above-described filer having a directional coupler or the above-described composite filter device having a directional coupler. Thus, a communication device having a superior transmission characteristic is easily formed.